Christina Aguilera plays mom during the day, but there's still room for Xtina at night.

A flawless-looking Aguilera strutted her way into blogger Perez Hilton's 31st birthday party at the Viper Room in West Hollywood on Saturday night.

This is the first night of partying we've seen from the blonde in a bit, as she's been holed up in the studio.

In addition to raising son Max Liron with her husband, music exec Jordan Bratman, she's working on a follow-up LP to 2006's Back to Basics.

She's recruited Australian singer Sia (of Six Feet Under's "Breathe Me" fame) for the new, as yet untitled, record.

As evidenced by what she wrote on her blog, Sia is a HUGE Aguilera fan.

"I definitely pooped my pants when she first sang out loud in the room with us," 33 year-old Sia wrote in her blog. "I love her. She's a nice person. A good person. An awesome mama. An incredible talent. Incredibly professional. A feminist. A rad role model for women who want to believe we can have it all, do it all."